Is your Mac frozen? It won’t respond and the mouse doesn’t move? Here is an easy way to unfreeze your Mac and get it going again.
Either click on your Apple menu in the top left corner and choose Force Quit or press the command, option and escape (esc) keys on your keyboard at the same time. The Force Quit menu will pop up.
Choose the application that is not responding by clicking on it until. This will cause it to be highlighted. In this example, Notes is not responding. Then click the Force Quit button.
This should close the frozen application and you can try to reopen it to use it again. You may want to restart your computer, just to be sure it did unfreeze your mac. If you would like to read more, try this article on Force Quit from Apple.